2 April 1954 399
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Fig. 2. Sailplanes were used extensively in wave research. This six-
minute flight record shows high-level turbulence encountered on one
fight: note the enormous changes in rate of climb.
howling. Several times you hang in your belt without the slightest
idea of attitude. You have not encountered anything like this
before. You recall a thunderstorm flight which scared you to death
but the turbulence was nowhere near this bad.
Suddenly you drop out of the cloud base and the view excites
you: everything seems to have changed. X-Mountain looks down
on you like a big barrier, the clouds sweeping down its slopes with
visible speed and dissipating just in front of you. You are about
ready to turn back when your plane is lifted with enormous power.
In heavy vertical gusts your rate of climb jumps to 1,000ft/min,
later to 2,000ft/min. The leading edge of the cumulus line is
now just above you. To avoid being pulled back into the roll
cloud you push the nose down. Apparently you now have a good
ground speed and the ship is climbing fast just in front of the
cloud line which looks like a long railroad train.
Suddenly the gusts die out. The air becomes smooth as glass.
But your rate of climb is now 2,500ft/min. You are stunned by
the fact that such extreme degrees of smoothness and turbulence
can co-exist so closely in the atmosphere. Looking back after a
few minutes you notice that you are already higher than the top
of the cloud. You are now flying at a safe level. That should be
enough finally to cross X-Mountain and the cap cloud. Your
altitude is 3,000ft over X-Mountain and probably 2,000ft over the
cap cloud. There is no roll cloud line ahead now and you have
reason to believe that you are out of trouble.
The foot of X-Mountain lies just below you. The trailing edge
of the cap cloud is only one mile ahead. The cloud mass pouring
down the mountain slope and dissipating is a fascinating spectacle.
The upwind edge of the high lenticular cloud is directly overhead,
maybe between 30,000 and 40,000ft.
The ship makes good headway now but the up-draught is
tapering off and you need more power to keep altitude and
ground speed.
High as you are above the low-level clouds you feel almost—
but not quite—safe. This completely smooth air has proved
treacherous before and you are not sure what it has in store for
you this time. The crestline of the mountain is not yet passed and
ground speed seems to drop again. After another minute the low
clouds look nearer. There has been no indication of what your
altimeter and rate of climb now reveal: you are falling again at
l,000ft/min, and full throttle does not help. You feel if you can
go another mile upwind you should be through.
But once more there is this unfortunate combination of a jetlike
headwind and a strong downdraught. You have been running
through several consecutive up and down draught areas. This is
indeed the pattern of an atmospheric wave. In another minute
you will know if you can pass X-Mountain. The cloud waterfall
is directly beneath the plane now.
But in front of you the cap cloud climbs fast. The air is still
quite smooth, but now you are falling at about 3,000ft/min. Three
thousand feet per minute? That means you will crash into the
mountain within another minute. What does your altimeter show?
A thousand feet above the highest peak of X-Mountain. But now
you can see a mountain peak through the cap cloud. That is cer
tainly not 1,000ft below you. It is just about your present height.
Altimeter wrong? Only a quick decision will save you. Turn back.
While you bank in a steep left turn the air becomes hazy. A
glance at the instrument panel and the mountains shows that you
are falling at almost 4,000ft/min into the lower end of the cloud
waterfall. Suddenly a terrific gust banks the airplane into a steep
right turn towards the mountain. For a moment you see the rocks
of the mountain rapidly coming nearer. Then you succeed in
manoeuvring the plane away from the stone wall.
You are right in the foot of the cloud waterfall which looked so
smoorh from above and the airplane shoots with an enormous
tailwind 1,500ft over the valley floor. As the heavy gusts diminish
you look back on the towering mountain range and the cap cloud
which only a few minutes ago lay under your feet.
In a matter of moments you have passed under the two roll
clouds and the nightmare is over. You decide to do what you
should have done in the first place: change your flight course,
flying around X-Mountain and a full-scale "Mountain Wave."
Wave Investigations. In order to investigate this type of airflow,
the "Mountain Wave Project" was commenced, sponsored jointly
by the Geophysics Research Directorate of the Air Force Research
Centre, Cambridge (Massachusetts), and the U.S. Office of Naval
Research. It was conducted by the University of California Soar
ing Association, together with a number of Government and
private organizations, and the field tests were made during 1951-52
in the Sierra Nevada mountain range in California.
Conditions of temperature, pressure and wind were investigated
up to a record height of 44,500ft by the use of specially instru
mented sailplanes, which were tracked by radar, Raydist, and
cinetheodolites. Time-lapse cameras took motion pictures of the
associated cloud structures from the ground, and meteorological
stations were established on both sides of the mountain range up
to an elevation of 9,000ft.
It was found that the phenomenon of the mountain wave is
essentially the same as the flow of water over a barrier which
forms rapids and waves downstream, but with added comp'ica-
tions due to atmospheric variables such as temperature, humidity
and wind. The troposphere shown in Fig. 1 consists of two layers
separated by a temperature inversion on top of the cap cloud. At
least two processes work simultaneously: (1) a "spill-over" of the
lower layer which shoots down the mountain slope with increasing
speed after passing the crest, and (2) an internal lee-wave in the
upper layer which forms in the wake of the mountain barrier.
The following conditions favour the formation of waves: —
(a) Wind flow perpendicular to the mountain range line and
with a speed of more than 25 kt at mountain top level.
(b) A wind profile showing a strong consistent flow extending
several thousand feet above the mountain tops, or showing an
increase in speed with altitude.
(c) An inversion or stable layer somewhere between the
mountain tops and the 600 millibar level.
The down-draughts to the lee of the rotor, and the up-draughts
below it, can carry a plane into the rotor cloud while a pilot is
attempting to pass above or below this cloud. If the aircraft
approaches the crest of the mountains from the downwind side
with insufficient height, it will be practically impossible to climb
through the air currents near the mountain slope. These condi
tions plus the fact that the peaks are hidden most of the time
make it probable that an aircraft fighting strong headwinds at
minimum clearance altitude would fly into the mountain peaks.
As the barometric pressure is considerably disturbed in the
mountain wave, altimeter errors are associated with the wave con
ditions. The maximum total error possible (giving a high reading)
has been estimated to be about 1,000ft, but errors as much as
2,500ft near the mountain peaks have been claimed by pilots.
On some occasions, when meteorological conditions are favour
able for the creation of a mountain wave, the lack of moisture in
the atmosphere can prevent the formation of clouds. The main
danger of this cloudless or "dry" wave is that it lacks the warning
features provided in most waves by recognizable clouds. More
serious still is the case where the wave flow is completely obscured
by a thick overcast with a low ceiling.
The following rules of flight are suggested to pilots for flights
over mountain ranges when wave conditions exist: (a) If possible,
fly around the wave area. If not, fly at least 50 per cent higher
than the height of the mountain range, (b) Do not fly high-speed
aircraft into the wave; particularly, do not fly downwind. Struc
tural damage may result, (c) Avoid the rotor (roll) cloud, (d)
Avoid the cap cloud (foehnwall)* area with its strong down-
draughts, (e) Avoid high lenticu'ar clouds if the edges are very
ragged and irregular, particularly if flying high, (f) If flying against
the wind, up-draughts areas, especially the one upwind of the rotor
clouds, may be used as an aid in gaining the altitude necessary to
pass through the down-draught areas and cross the mountain
range, (g) Do not p'ace too much confidence in pressure alti
meter readings near the mountain peaks, (h) Avoid penetrating
a strong mountain wave on instrument flight.
*"Foehn" is a meteorological term for the air current descending from
a mountain range.
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